How is the reCaptcha useful for Google?
At this time, Google is aggressively pursuing research into AI of many sorts. All AI at this point is constrained by the size and accuracy of available training data. Getting human curated data is expensive and time consuming. Crowdsourcing through Captcha gets access to that without paying directly. 

Why roads specifically? It can hardly be a coincidence that Google's holding company also owns a self driving car company Waymo. They need machines to learn what road signs look like in real life, so that they can respond to them.